Resistor

ABSTRACT

SEMICONDUCTORS USEFUL AS ELECTRICAL RESISTANCE HEATING ELEMENTS AND COMPOSED PRINCIPALLY OF ZINC OXIDE HAVE A RESISTIVITY VALUE NOT HIGHER THAN TEN OHM-CM. AT 25*C. AND NOT HIGHER THAN ONE OHM-CM. AT 925*C., THE RATIO BEING BETWEEN ABOUT ONE AND ABOUT TEN.

Dec.l 28,-1971 K, E, NELSON 3,630,970

l REsIsToR v v A i Y Filed June 24, 1968 United States Patent lce U.S.Cl. 252-518 8 Claims ABSTRACT OF THE DISCLOSURE Semiconductors useful aselectrical resistance heating elements and composed principally of zincoxide have a resistivity value not higher than yten ohm-cm. at 25 C. andnot higher than one ohm-cm. at 925 C., the ratio being between about oneand about ten.

This invention relates to electrical resistors, and more specifically toresistors composed principally of zinc oxide. In one important aspectthe invention relates to ceramic zinc oxide resistance heating elementsand to `space heaters incorporating the same; and the invention will forconvenience be described primarily in terms of such elements.

The novel electrical resistance heating elements of this invention arecharacterized as having a resistivity value at normal room temperature(25 C.) of less than about ten ohm-cm. Preferred structures have a roomtemperature resistivity value not higher than about one ohm-cm. and manyare well below that value. The resistivity value at the preferredoperating temperature of about 925 C. (1700 F) is in all cases less thanone ohm-cm., and the ratio of the resistivity value at 25 C. to that at925 C. is between about one and about ten. The resistivity value isdefined as the resistance in ohms between opposite faces of a 1 x 1 x 1cm. cube of the material.

Another important characteristic of the resistance heating elements ofthe invention is their ability to retain substantially constantresistivity value under prolonged operation at high temperature.Furthermore these elements may be made to show a high degree ofresistance to mechanical and thermal shock.

These several properties make possible the production of ceramicsemiconductor electrical resistance heating elements which are capableof being rapidly brought to full operating temperature and of then beingstably maintained at that temperature during prolonged periods ofoperation at constant voltage, and which are sufficiently rugged towithstand all ordinary handling, vibration, and thermal shock.

Resistors and resistance heating elements comprised principally of zincoxide have previously been described. In U.S. Pat. No. 2,892,988,elements containing at least 60% of zinc oxide are given a positivetemperature coefficient of resistance by the inclusion of nickel oxideor titanium dioxide, whereas oxides of such metals as zirconium,beryllium, iron, aluminum and copper contribute to a negativecoefficient. Although Iired at 900 to 1400 C. during manufacture, theelements are used only at much lower temperatures, for example at 300-00 C.

Titanium, nickel, magnesium and zirconium oxides are combined with atleast 70% of zinc oxide as described in U.S. Pat. No. 2,933,586 toproduce resistance heating elements having a positive temperaturecoefficient of resistance within the suggested operating range of 200 to500 C. Oxides of titanium and nickel are suggested for the same purposein U.S. Pat. No. 3,037,942 which provides additional details as to thebehavior of these compositions at the lower and higher ends of thetemperature range, where a negative temperature coeliicient ofresistivity is experienced.

It has now been found possible to overcome these and other limitationsof prior art resistance heating elements,

3,630,970 Patented Dec. 28, 1971 and to produce resistor structureswhich heat up rapidly and which may be operated at much highertemperatures, e.g. of the order of 925 C., by incorporating with highpurity zinc oxide certain metal oxide modifiers -in specific smallproportions and in a manner all as will be hereinafter described anddefined.

The practice of the invention has been found particularly useful in theproduction of resistance heating elements for domestic space heaters orradiant heaters operating from conventional household electric powersupply outlets. One form of such a device is illustrated in theaccompanying drawing, wherein:

FIG. 1 is a side elevation, partly broken away to show linteriordetails, of an overhead heater unit,

FIG. 2 is a similar bottom plan view of the unit of FIG. l,

FIG. 3 is a sectional elevation approximately along line 3 3 of FIG. 1,

FIG. 4 is a plan view of a resistance heating element as used in theunit of FIGS. 1-3, and

FIG. 5 is a sectional view taken along the line 5 5 of FIG. 4.

The heater unit of FIGS. 1-3 comprises an elongate box-like open-facedcasing 10 supported from a section of conduit 11. Within the casing ismounted a frame 12 carrying a rounded elongate trough-like reflector 13and a pair of high-temperature-resistant insulative support members 14each having a clamp-like contact element 15, the latter being connectedto electric power wires 16 entering through the conduit 11 and passingalong the open channel provided by the upper U-beam element 17 of theframe 12. An open protective wire grid 18 covers the open face of thecasing.

The resistance heating element 20 is suspended centrally of thereflector 13 and is connected to the electrical circuit by the metalclamps 15. The element 20 is in the form of an elongate thin-Walled tubeas shown in FIGS. 4 and 5, and is provided at each end with a noblemetal conductive coating 21. Effective contact is assured by wrappingthe contact areas with one or two turns of a resilient metal screen orthe like and by tightening the clamp 15 by means of the screw 22.

In a specific example the heating element 20 is 18 inches `in length,3A; inch in outside diameter, and has a wall thickness of /g inch.`Silver metal is used as the conductive coating 21, the silver extending11/2 inches along each end of the tube. leaving an active length of l5inches between contacts. The unit is designed to operate at -115 voltsAC and at that voltage reaches full temperature within about 11A-2minutes and then remains at a surface temperature Very close to 925 C.The temperature -is conveniently measured by means of a thermocoupleinserted within the hollow interior of the tube.

Despite published resistivity values to the contrary, extremely pure`zinc oxide has been found capable of yielding heating elements of theform shown in FIGS. 4 and 5 having resistivity values less than tenohm-cm. As an example, pure zinc oxide containing less than ten partsper million of alkali metal oxides, formed into rods of 1A inch diameterand sintered at 1300-l400 C., has shown resistivity values of 0.4 to 0.7ohm-cm. Rods or tubes made with pure zinc oxide are found to be fragileand to break or disintegrate when roughly handled or when subjected tovibration, and for this and other reasons have not been found acceptableas resistance heating elements for space heaters as hereinbeforedescribed.

It has now been found possible to toughen and strengthen Zinc oxideresistance heating elements while maintaining or improving their lowelectrical resistivity properties. At the same time it has been foundthat processing control is creativ simplified. so that duplicateelements of substantially identical properties may be made.

In addition, elements of greatly improved electrical and thermalstability are provided. These and other advantageous properties andresults are achieved, in accordance with the principles of thisinvention, by incorporating with the pure zinc oxide minor quantities ofzirconia and/or silica, preferably together with trace amounts of oxidesof aluminum, indium, gallium, iron, or mixtures thereof, as will befurther described and illustrated.

The incorporation of zirconia in particular, and of silica to a lesserextent, is ffound to toughen the resulting article to an extentsufficient to prevent breakage under all ordinary handling, and elementsof such composition may be packaged, shipped, installed, and subjectedto moderately severe vibration without damage. In addition thesecomponents contribute greatly increased thermal shock resistance, to theextent that tubes of the material heated to bright red heat may besafely sprinkled with droplets of water without cracking. At least aboutone or two percent of zirconia is ordinarily required to provide asatisfactory degree of ruggedness. Above about 8% of SiO2 or neutralizerfor trace amounts of alkali metal oxides increases tremendously,apparently due to an inversion of phase, and at the same time thephysical structure is weakened; so that the amounts of these twocomponents are to be held below the approximate percentages indicatedand within the range wherein the zinc oxide remains as the continuousphase in the ceramic product.

Either silica or zirconia will provide the advantages just noted. Anadditional advantage is contributed by silica, either alone or inconjunction with zirconia; and for this purpose very small amounts ofsilica are useful. It is found that silica has the ability to act as ascavenger or neutralizer for trace amounts of alkali metal oxidesintroduced as impurities in the zinc oxide or in other ways. The alkalimetal oxides even in extremely small amounts are found to cause verysubstantial and usually harmful increase in resistivity values. Sincetrace amounts of sodium salts in particular are frequently present, orare added as unavoidable impurities during compounding, the inclusion ofsmall proportions of silica is ordinarily desirable in order to maintainthe required low resistivity value without unduly increasing theprocessing costs.

The addition of trace amounts, as before indicated, of oxides ofaluminum, iron, indium and gallium is effective in decreasing theresistivity of the zinc oxide heating elements, and to that extent thesematerials operate in a direction opposed to that of the silica, zirconiaand alkali metal oxides. The amounts of these additives must beextremely small, i.e. not above about .03 percent, in order to maintainstability of resistivity during operation of the elements at hightemperature. Trace amounts of the order of only a few parts per millionof these materials are ordinarily fully adequate in obtaining therequired low resistivity values. Small amounts of iron oxide decreasethe resistivity of the zinc oxide sinter at high temperatures butincrease the low temperature values; the inclusion of traces of silicain such composition reduces the low temperature resistivity.

The well-mixed finely powdered oxides or oxide-forming components may becompacted in dry form under high pressure and then fired at hightemperature. The preparation of shaped resistance heating elements suchas the tubular element of FIGS. 4 and 5 is preferably accomplished byfirst pre-mixing the oxides or the like in aqueous suspension; drying toa uniform powder; blending the powder with sufficient pure Water, andwith small amounts of wetting agents, lubricants, binders and othertemporary additives as desired, to form a paste of proper plasticitycharacteristics; and then extruding or otherwise forming the mass intothe appropriate shape under high pressure, drying the green article, andfiring at sintering temperature. The resulting resistance element isthen desirably further stabilized by prolonged heating at a temperaturesomewhat below the sintering temperature but above the proposedtemperature of operation, and for a time sufficient to provide a stableresistivity value characteristic.

Although the composition may be formed directly from the oxides, it isfrequently more convenient to use other and more readily available ormore easily stored or mixed compounds as the source of the metal oxides.For such purposes, decomposable salts such as the nitrates or thecarbonates may be employed; but compounds containing halogen ion inparticular are to be avoided as having an inhibiting effect on thesintering and as preventing attainment of maximum density in thesintered product.

As an illustration of the difficulties involved in maintaining freedomfrom alkali metal oxides in these compositions, it has been observedthat commercial methyl cellulose, commonly employed as a temporarybinder for such ceramic materials, may contain as much as 1000 to 2000parts per million of sodium chloride. The presence of this material actsboth to increase the resistivity and to inhibit sintering anddensification, and is to be avoided. However in small amounts the sodiumion may be effectively scavenged by the silica content of thecomposition, as previously noted; and the limited amount of chlorinepresent in such compositions is not ordinarily sufficient to be harmful.Prolonged extraction with hot distilled `water has been found useful inreducing the sodium chloride content of methyl cellulose toinsignificant levels of less than ten parts per million.

Somewhat analogously, the mixing or grinding or other processing of thecomponents or mixtures may result in the incorporation of unmeasuredsmall but significant amounts of iron, aluminum or other metalliccomponents which affect the properties of the completed resistor. Suchmaterials must therefore be avoided by proper selection of processingequipment and conditions, or the composition must where possible beproperly formulated to compensate for their addition.

Although active impurities in general and alkali metal oxides inparticular are to be avoided in the compositions and resistance elementsof the invention, it is nonetheless found possible to incorporate smallamounts of inert metal oxides, e.g. cobalt and nickel oxides, withoutsignificant effect on mechanical or electrical properties and withspecific advantages. As an example, from .005 to .5 percent of cobaltoxide, or somewhat larger proportions of nickel oxide, provide permanentcoloration of the piece and are useful in color coding or for decorativeeffects.

The application of noble metal coatings to serve as contact areas iswell known in the industry and need not be detailed here. Coatings of.silver are satisfactory in most instances, but gold coatings may beemployed where higher temperature operations, up to about l050 C., areindicated, and platinum and palladium may be used for still highertemperatures.

The following examples, in which all proportions are given in parts byWeight unless otherwise indicated, will serve further to illustrate thepractice of the invention which however is not to be limited thereby.

EXAMPLE 1 The inherent instability of zinc oxide resistors containingmore than the indicated proportions of modifiers is indicated by theresults obtained using pure USP zinc oxide blended with 0.75 weightpercent of aluminum oxide. The mixture is formed into a resistanceheating element under pressure and is tired for one hour at 1385 C. Thesintered element is then further heated for 19 hours at 1025 C. Afterapplying metallic terminals, the element is connected to a source ofelectric power through a suitable voltage control device. With thevoltage initially at 64 volts, the temperature of the piece risesrapidly to 925 C. During a period of ten days it is found necessary toincrease the voltage gradually to volts in order to maintain the '925 C.temperature. The voltage is then held at 90 volts for a further 2O days.After remaining at 925 C. to the fourteenth day, the piece graduallycools, reaching 800 C. after a total of 30 days. At that point thevoltage is again increased in an effort to Sample l is again fragile andeasily broken, whereas samples 2-6 are resistant to both mechanical andthermal shock. The resistivity values at 925 C. are in each instancesomewhat lower than those at room temperature.

EXAMPLE 4 The effect of small amounts of other modifiers is shown inthis example, wherein is reported the resistivity values obtained atroom temperature and at 925 C.

ZnO Zl'Oz SiOz A1203 111203 G3203 F6203 R25 R925 order to maintain theinitial temperature; at which point thermal run-away occurs.

Such resistors are not amenable to continuous operation at hightemperature under fixed voltage, since the temperature either slowlydecreases or rapidly increases to a point at which failure of the unitoccurs.

EXAMPLE 2 The following compositions are prepared and formed into testelements by the preferred process, using'USP grade zinc oxide powderwhich by analysis is shown to contain no more than three parts ofaluminum oxide, iive parts of silica, one part of ferric oxide, and lessthan three parts of alkali metal oxides, per million parts of thepowder. The dried compressed bars are heated in air for one to two hoursat 1400 C. and are measured for resistivity. Resistivity (R) is reportedin ohm-cm. at the subscript temperature.

Under the same conditions, the room temperature resistivity of anelement prepared from the USP zinc oxide without modifiers is 0.45ohm-cm. at room temperature and the sample is less rugged and moreeasily broken than are those samples containing two or more parts ofsilica.

When the proportion of silica is progressively increased much beyondthat of sample No. 4 the resistivity value rapidly increases to a levelat which the product is no longer practical as an electrical resistanceheating element` A similar result is experienced when the concentrationof soda is increased in the absence of silica, as indicated by theresistivity values reported in samples and 6.

EXAMPLE 3 Samples 1 4 and 6 are prepared by wet mixing, drying,remoistening with water and glycerine, compressing, drying, andsintering at 1400 C. The remoistening is omitted in the preparation ofsample 5, the dry powder being pressed into a preform suitable forsintering by pressures of 5,000 to 10,000 p.s.i. The use of solublesalts of the trivalent metals, e.g. gallium nitrate in sample 5, permitsthorough blending of the components and provides a homogeneous article.Samples 2-6 are stabilized by further heating for 48 hours at 1025 C.

Samples l and 6, containing two percent of zirconia, are strong andrugged, being highly resistant to both mechanical and thermal shock andforming very acceptable electrical resistance heating elements. Samples2-5 are less rugged, and samples 3-5 are relatively fragile, but insmall sizes may be used as resistors and for other applications wherethey are not subjected to shock. The physical properties of thesesamples may be improved by the incorporation of zirconia as in samples 1and 6.

EXAMPLE 5 A composition containing 98 parts of zinc oxide, two parts ofzirconium oxide, .03 part of silica and .003 part of alumina, asdescribed also under sample 1 of Example 4, is processed by wet mixing,spray drying, remoistening with water containing glycerine and purifiedmethyl cellulose, compacting by extrusion, drying, and firing, toproduce a tubular electrical resistance heating element which is coatedat both ends with silver paste to provide contact areas and is thentested for resistivity.

The initial firing is for three hours at l400 C., after which treatmentthe tube shows a resistivity of .04 ohmcm. over the entire range of 25to 925 C. The tube is then heated an additional 60 hours at l050 C.Thereafter the resistivity at 25 C. is .3 ohm-cm. and at 925 C. is .1ohm-cm.; and this resistivity is maintained during longcontinued use ofthe tube as an electrical resistance heating element at temperatures inthe neighborhood of 925 C.

Heating at just below sintering temperature and above subsequentoperating temperature for a time usually of the order of 30 to 60 hoursis effective in bringing the resistivity value to a constant maximum ineach of the compositions of the present invention.

A tubular heating element prepared as above described and having anoutside diameter of inch, a wall thickness of 1/16 inch, and a lengthbetween electrodes of 15 inches, is particularly suitable for use in thespace heater described in connection with the appended drawings. Thetube has a resistance of about 9.6 ohms, draws 1500 watts at volts, andoperates at a temperature of approximately 925 C. Tubes of otherspecific dimensions may easily be designed for operation at othervoltages or temperatures where desired.

EXAMPLE 6 Whereas the oxides of aluminum, gallium and indium may serveas sole modifiers of pure zinc oxide in the preparation of lowresistivity elements, iron oxide is effective for such purposesprimarily in conjunction with silica, as is shown in the followingcomparison which illustrates also the incorporation of zirconia toprovide additional physical ruggedness. The samples are sintered onehour at 1400 C. and stabilized by heating for several days at 925 C.

F0203 R25 Rm What is claimed is as follows:

1. A ceramic semiconductor resistance element consisting essentially ofzinc oxide together with modifiers making up about .002 to about 30percent of said element and including about one to thirty percentzirconia, about .O05 to about one percent silica, and 0-.03 percent ofoxide of at least one of the trivalent metals aluminum, gallium, indiumand iron, said element being further characterized as having aresistivity value at 25 C. of less than about ten ohm-cm., a resistivityvalue at 925 C. of less than about one ohm-cm., and the rati ofresistivity value at 25 C. to that at 925 C. being between about one andabout ten.

2. The resistance element of claim 1 wherein the resistivity value at 25C. is less than about one ohm-cm.

3. The resistance element of claim 1 wherein is included about one toabout ve percent of zirconia.

4. The resistance element of claim 1 wherein is included about .002 toabout .03 percent of said trivalent metal oxide.

5. The resistance element of claim 1 wherein is ineluded about one toabout tive percent of zirconia, about .005 to about one percent ofsilica, and about .002 to about .03 percent of said trivalent metaloxide.

6. The resistance element of claim 5 wherein the modiers are about twopercent of zirconia, about .03 percent of silica, and about .O03 percentof said trivalent metal oxide.

7. The resistance element of claim 6 wherein the trivalent metal oxideis aluminum oxide.

8. The resistance element of claim 1 wherein is included at least aboutone percent of zirconia, at least about .005 percent of silica, and atleast about .001 percent of alumina, said element being in the form ofan elongate tube characterized by resistance to mechanical and thermalshock and by having a substantially constant resistivity value duringprolonged heating at 925 C.

References Cited UNITED STATES PATENTS 2,887,632 5/1959 Dalton 252-5123,089,856 5/1963 Cyr et al. 252-518 3,264,229 8/1966 Klein 252-5182,892,988 6/ 1959 Schusterius 252-520 2,933,586 4/1960 Schusterius252-520 3,509,073 4/1970 Bowman 117-201 3,515,686 6/1970 Bowman 252-518OTHER REFERENCES Chem. Abstracts, vol. 6l, 2585b, Valleer et al., 1964.

DOUGLAS I. DRUMMOND, Primary Examiner U.S. C1. X.R. 10G-296; 23-147NTTED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,633197) Dated DeCembeI' 28 1971.

Inventor(s) Karl-E NelSOD It is ertified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown`belw:

CO1 3, line 2l.

c I o l "or neutralizer for' trace amounts of alkali metal oxides' Lshould be "or about 30% of ZTO2 the resistivity of the material" Signedand sealed this 8th day o August 1972;

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents F ORM FO-105O (1G-69) USCOMM-DC 603764560 w u.s. GoyERNMENTPRINTING office |969 o-:ss-au

